There has been disclosed an internal combustion engine in which CNG (compressed natural gas), which is hard to be ignited, is used as a main fuel, and light oil, which is easy to be ignited, is used as an auxiliary fuel (for example, refer to a first patent document). In this internal combustion engine, when an operating state of the internal combustion engine is under light load, only light oil is used as fuel, but when it is under medium to high load, both fuels, CNG and light oil, are used. In addition, in cases where both the fuels, CNG and light oil, are used, light oil of necessary amount for ignition is used as the auxiliary fuel which is easy to be ignited so that CNG as the main fuel which is difficult to be ignited is caused to combust by igniting the light oil.